Printed circuit board assemblies are cooled after reflow soldering to reduce the temperature of the soldered articles below the melting temperature of the solder. In reflow soldering the articles to be soldered are first covered with a solder paste containing flux which is then heated in at least one heat zone so that the solder melts and the flux liquifies permitting the solder to flow and cover the joint or area to be soldered. After heating, the soldered articles pass into a cooling section where the solder is cooled below the melting temperature, thus hardening the solder on the circuit board assemblies. In most cases there is also some liquid or solid flux deposits on the solder which forms in the cooler section.
One example of a reflow soldering apparatus is disclosed in U.S. Pat. No. 5,125,556 to Deambrosio and one example of a cooling unit for a reflow soldering system is disclosed in U.S. Pat. No. 4,912,857 to Parent et al. The cooling unit is generally a separate section and incorporates moving ambient gas via fans or blowers through a heat exchanger. The recirculation of the ambient gas can cause problems with regard to flux deposits in the heat exchangers and within the flow actuators. These deposits can cause clogging of the heat exchangers and flow actuators which can degrade the cooling performance over time. This results in increased maintenance and down time.
Various methods of reducing flux deposits in the cooling section have been attempted. One remedy is a filtration system wherein gases in the reflow heating zones are passed through a filtering medium before entering the cooling zone. This technique is not always effective as it is difficult to filter out the flux components when in the vapour phase. Thus, filter systems may slow the accumulation of flux deposits in the cooling zone, but they do not solve the problem.
Another suggested scheme is a cleaning cycle for the entire reflow apparatus. In this system, the heating zones and the cooling zones are heated to a temperature which allows flux deposits within the oven to vaporize. Such a procedure, however, has a number of problems, one of them being due to the large thermal mass of the heating zones requiring high heating energy which is both costly and time consuming. There is a three step process which heats up, bakes out and cools down the complete apparatus. This requires the shut down of the reflow apparatus. Another problem is that the recirculating coolant must be entirely purged from the heat exchanger used in the cooling zone, otherwise it may rupture as a result of high pressure at elevated cleaning temperatures. This last point is of concern because a failure of the purge system could result in personal injury.
In most cooling sections of a reflow soldering apparatus, a high circulation of ambient gas passes through a heat exchanger to cool the gas. The soldered articles are then conveyed through the cooled gas. The two most common gases used in this cooling system are air and nitrogen. Nitrogen provides an inert process environment and much brighter solder joints are obtained in a nitrogen atmosphere as oxidation does not occur on the surface of the solder.